Since 3,4-dihydroxyphthalic acid is a useful monomer as a raw material of various functional polymer materials, it is used in various fields of chemical industries.
To introduce hydroxyl groups specifically to the 3- and 4-positions of phthalic acid is very important in polymerization for the use of this material (3,4-dihydroxyphthalic acid). This is because, if only a small amount of a by-product in which hydroxyl groups have been introduced onto the 3- and 5-positions is existing with 3,4-dihydroxyphthalic acid, the polymerization reaction is stopped by that by-product, and a material having a satisfactory molecular weight and molecular length cannot be obtained, thereby increasing the possibility that the desired function will be adversely influenced.
To produce 3,4-dihydroxyphthalic acid, a chemical synthesis technique is taken into consideration, but the synthesis process is complicated. In particular, in the synthesis, it is difficult to introduce hydroxyl groups specifically to the adjacent 3- and 4-positions. By a method wherein the desired product is obtained as an intermediary metabolite by biodegrading phthalic acid with a microorganism, the desired product wherein hydroxyl groups have been introduced specifically onto the 3- and 4-positions can be obtained. However, the method has a problem due to living cells, which is that further decomposition occurs owing to an enzyme present in the living cells. Therefore the particular substance desired cannot be obtained efficiently. The microorganism capable of converting phthalic acid to 3,4-dihydroxyphthalic acid is described in Eaton, R. W. & Ribbons, D. W. (1982) J. Bacteriol., 151, 48-57.
Accordingly, under the existing circumstances, a method for efficiently obtaining only 3,4-dihydroxy-phthalic acid is needed.